The present invention relates to the refining of fibrous material, such as wood chips, and particularly relates to novel and improved methods and apparatus for refining comminuted fibrous chips, e.g., refining chips to pulp according to thermomechanical pulping (TMP), chemimechanical pulping (CMP), and chemithermomechanical pulping (CTMP) or other methods for the production of high-yield or mechanical pulps.
In mechanical pulping, chips are broken down into progressively smaller chips or pulp using a refiner, i.e., a defibrator. Typically, a refiner or defibrator includes relatively movable grinding surfaces defining a grinding zone therebetween wherein chips are reduced to form pulp. These grinding surfaces, for example, may comprise oppositely disposed discs or conical surfaces and are relatively rotated by an electric motor. In areas where electrical costs are high, the cost of operating the refiner can be prohibitive. For example, the energy conversion can be on the order of magnitude of approximately 1000 kWh per ton of pulp.
It has been recognized that only a small portion or fraction of this electrical energy is used to defibrate the chips. The major portion of the energy is used to convert the moisture in the fibrous material, as well as any liquid added in the defibration process, into steam. Steam, however, undesirably occupies the majority of the space within the grinding zone. The presence of steam in the grinding zone is undesirable because it inhibits the natural passage of the fiber material through the grinding zone and decreases the retention time of the fibrous material in the grinding zone.
In previously filed U.S. patent application Ser. No. 8,667 filed Jan. 30, 1987, now abandoned, and in co-pending application Ser. No. 70,212 filed July 6, 1987, the problem of reducing the energy costs in refiners of this type is addressed. In such applications, there is described a method for lowering energy consumption during refining of fibrous material by decreasing the grinding frequency through a reduction of the number of cutting elements on the grinding surfaces. This, in turn, increased the fiber material retention time in the grinding zone. Using cylindrical or frusto-conical grinding zones as well as removing a substantial portion of the steam from the grinding zone further improved the fiber material retention time.
According to the present invention, there is provided a refiner apparatus wherein the grinding zone is defined between a pair of cylindrical or preferably frusto-conical grinding surfaces wherein the fiber material retention time in the grinding zone is increased by the removal of an increased quantity of the steam produced in the grinding zone in a simple, efficient and practical manner. In a manner to be explained, the steam generated in the grinding zone is removed such that the available volume in the grinding zone for refining material is increased and the volume occupied by the steam is substantially decreased and minimized. The removal of increased quantities of steam is accomplished efficiently and in such manner that fibrous particles following the steam in the process of its removal from the grinding zone are passed through a multi-stage separation process. This multi-stage separation is accomplished within the confines of the refiner itself, using the rotary action of the refiner rotor to achieve separation of the removed steam and any fibrous material particles following the steam during its removal. Thus, the multi-stage separation process may comprise a pair of centrifugal separators using the rotor of the refiner as the driving force for effecting separation and with the parts thereof arranged to effectively recombine the fibrous materials removed from the separation stage with the fiber material outlet of the grinding stage. The steam is therefore removed in a relatively clean condition without the necessity of additional cyclones or other separation apparatus.
According to one aspect of the present invention, there is provided apparatus for refining fibrous material including a housing having a first grinding surface, a rotor carried by the housing for rotation about an axis and having a second grinding surface, the grinding surfaces defining a grinding zone therebetween, and means for rotating the rotor. The housing includes an inlet to the grinding zone for receiving coarse fibrous material, such as wood chips, and an outlet from the grinding zone for removing ground fibrous material. A centrifugal separator is carried by the rotor and includes an interior chamber defined by the rotor and disposed inwardly of the second grinding surface. Means are carried by the rotor for removing steam generated in the grinding zone and directing the steam inwardly of the rotor into the chamber. The separator has a fibrous material particles outlet and a steam outlet in communication with the chamber. Steam and any fibrous material particles contained therein and following the steam from the grinding zone into the chamber are separated one from the other in the separator by centrifugal action for flow through the steam outlet and the separate particles outlet, respectively. Preferably, the steam removal means includes a plurality of openings formed in grinding segments mounted on the surface of the rotor for directing the steam from the grinding zone inwardly into the chamber. The chamber is in part defined by the inside wall of the rotor and has a plurality of circumferentially spaced ribs for directing the separated fiber material particles through an outlet adjacent the end of the rotor in communication with the outlet for the fiber material from the grinding zone.
Preferably, a second-stage separation is also provided and which lies in communication with the steam outlet from the chamber of the first separation stage. The second separation stage includes a separation zone between the rotor and the housing inwardly of the chamber and ribs carried on an inside surface of the rotor wall for directing fibrous material particles to the previously mentioned particle outlets.
It is also a feature of the present invention that the grinding zone may be increased or decreased in width in a relatively simple, practical and efficient manner. Particularly, the housing includes end parts mounting an intermediate part therebetween for axial displacement, the intermediate part carrying the inwardly directed grinding surfaces in opposition to the grinding surfaces of the rotor. The intermediate part is thus axially displaceable, for example, by means of a hydraulic cylinder, to increase or decrease the spacing between the grinding surfaces of the intermediate part and the rotor.
In a still further embodiment of the present invention, there is provided a method of refining fibrous material including the steps of grinding the fibrous material in the grinding zone between the grinding surfaces whereby steam and fibrous material particles are generated in the grinding zone, flowing at least a portion of the steam and fibrous material particles following the steam through openings in the rotor into an interior chamber formed by the rotor, separating the particles in the steam in a first separation stage in the chamber, flowing at least a portion of the steam and following fibrous material particles from the first separation stage into a second separation stage which is, in part, formed by a rotatable surface of the rotor and separating the particles and steam in the second separator stage.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for refining fibrous material which effectively reduces the energy costs of converting the fibrous material to pulp by effectively and efficiently removing steam from the grinding zone to increase fiber material retention time in the grinding zone and to separate from the removed steam any following steam particles for recombination with the ground fiber material at its outlet and providing clean steam.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.